Sporic meiosis life cycle occurs where the purpose of this cell division is spore production. The spores are haploid which will be produced by the diploid sporophytes. The process of spore production is known as sporogenesis.
These haploid spores are essential for the further sexual reproduction in algae. Algae which produces haploid spores from diploid cells is said to have a sporic meiosis life cycle.
Sporic meiosis life cycle in Rhodophyceae
All but a few species of sexually reproducing Rhodophyceae exhibit sporic life cycles and all are oogamous.
In Florideophyceae, the majority of species have a triphasic Polysiphonia-like life cycle. At the same time, this cycle is lacking in Palmariales and Bonnemaisoniales.
The dioecious gametophytes and the tetrasporophyte are most often isomorphic while the carposporophyte is reduced and remains attached to the female gametophyte.
The male gametes, which are aflegellate and lack cell walls are termed spermatia and are formed singly in each spermatangium. The oogonium (carpogonium) has a hair-like extension at the apex. During fertilization, the trichogyne receives the spermatium.
The location of the carpogonium and the post-fertilization changes in the development of the carposporophyte have been the accepted basis for the classification of the Florideophyceae at the ordinal level.
In the Nemaliales, the carposporophyte develops directly from the fertilized carpogonium, and a mass of carpospores is formed.
In other words, the zygotic nucleus is transferred to an auxiliary cell; the transfer may involve the development of a long connecting filament or ooblast. Secondary ooblast filaments may arise from the auxiliary cell and fuse with other auxiliary cells.
The auxiliary cells, which may fuse with other vegetative cells, ultimately produce a series of gonimoblast filaments which will later produce the carpospores.
The auxiliary cell and its derived cells make the carposporophyte. The carpospores when released will germinate to develop the diploid tetrasporophyte phase.
It will later produce tetrasporangium which on maturity produces tetraspores by meiotic division. In most of the cases, the sex is genotypically determined during meiosis.
Heteromorphic life cycles in Rhodophyceae
Some members of Rhodophyceae show heteromorphic life cycles. These usually involve the alternation of generations. Audouinella pectinatum shows a heteromorphic sequence of tetrasporophyte 1-2 cm high and dwarf filamentous gametophytes <0.5 mm high.
Other species of Nemaliales, Nemalion, Liagora, and Pseudogloiophloea have larger gametophytes with Audouinella-like tertrasporophytes.
The Cryptonemaiales and Gigartinales contain several genera in which the tetrasporophyte is reduced, usually to a crustose phase. Carpospores from Halymenia sp develop into an Audouinella-like phase.
Gigartina sp, have essentially Polysiphonia-life cycles. In Mastocarpus, carpospores may directly develop into carposporophyte-bearing female gametophytes.
Members of Phyllophoraceae show a wide variety of life cycles. In Gymnogongrus griffithsiae, carposporangia formed within the female gametophyte produces filaments that bear tetrasporangia on the surface of the thallus.
In Liagora tetrasporifera, the carposporophyte produces meiotic tetrasporangia at the apex of the gonimoblast filaments. Such carposporophytes are called carpotetrasporophytes.
In Palmariales, the female gametophyte is reduced and the carposporophyte is lacking. Tetrasporophytes and males are reported in Palmaria palmata, but female plants are unknown (because the female gametophyte is much reduced to a minute crustose form, which produces the tetrasporophyte after fertilization).
Members of Rhoymeniales and Ceramiales exhibit a Polysiphonian life cycle with many minor deviations usually as a result of failure of meiosis in the tetrasporangium.
Members of Bangiales display a reproductive cycle involving a shell-bearing alga Conchocelis rosea. This was shown in Porphyra umbilicalis.
The carpospores germinate to produce the filamentous Conchocelis (monospores recycle this conchocelis phase). This diploid phase produces concho spores through meiosis and these spores develop into the foliose Porphyra.
Sporic meiosis life cycle in Prymnesiophyceae
The sporic meiosis life cycle in this family is confusing for want of information on the site of meiosis. The alternation between a diploid free-living coccolithophorid form and a diploid benthic pseudo-filamentous stage has been reported.
Sporic Life Cycle in Phaeophyceae
A sporic meiosis life cycle involves the alternation of generations of a free-living haploid gametophyte generation with a morphologically similar or dissimilar free-living diploid sporophyte is characteristic of a sexually reproducing brown algae except for Fucales.
In Cutleriales, Zanardina has an isomorphic alternation of generation while Cutleria exhibits a heteromorphic cycle with a prostate sporophytic phase (Aglaozonia stage).
Marked anisogamy or oogamy is characteristic of the orders Cutleriales, Sphacelariales, Dictyotales, Laminariales, and Desmerestiales. The order Ectocarpales, Dictyosiphonales, and Chordariales are predominantly isogamous.
Ectocarpales, Tilopteridales, Dictyotales, Sphacelariales, and Cutleriales (partially) show predominantly isomorphic alterations, whereas alternation of heteromorphic phases occurs in other orders.
The gametangia of both isogamous and anisogamous species are plurilocular structures. These plurilocular sporangia may be similar and can be seen in some species where it produces spores.
In oogamous species, the oogonium produces only one egg while there are abundant male gametes in in one-chambered antheridia or plurilocular antheridia. Meiosis occurs in the unilocular sporangia borne on sporophytes.
Sporic life cycle in Chlorophyceae
Isomorphic sporic meiosis life cycles are common in Cladophorales and Ulvales, while heteromorphic ones are seen in Acrosiphonales and some members of Caulerpales. An alternation of generation occurs between a quadriflagellate, spore-producing sporophyte generation, and a monoecious gametophyte generation which produces biflagellate gametes.
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